Automatic transmissions have been in use for some time, in particular in motor vehicles, to convert an input speed supplied by a vehicle's prime mover and/or an internal combustion engine into an output speed used for driving the wheels, where the driver is relieved, at least for the most part, from selecting the appropriate gear ratio.
The gear ratios of these automatic transmissions can be infinitely variable or can have individual transmission ratios with predetermined gear ratios. For the most part, they are shifted by way of hydraulic shifting elements, which require an oil supply that has a sufficient volume and pressure for their operation. An oil pump, which is mechanically driven by the internal combustion engine of the vehicle, is commonly used for this function.
A gear ratio change, which automatically shuts down the vehicles when specific conditions are present, is frequently provided in modern drive concepts in order to minimize the consumption of fuel, as well as the emission of pollutants. This can be the case, for instance, during the “sailing mode”, when the vehicle is coasting, as well as during standstill, and is known under the term of a start/stop function. When it is determined that the driver wants to continue driving, or the driver again requests a driving torque from the internal combustion engine by actuating the accelerator pedal, the internal combustion engine is again started without requiring any further operating steps by the driver, a corresponding drive gear ratio is engaged or maintained within the automatic transmission, and the drive clutch is engaged.
Since an oil pump, which is driven by an internal combustion engine, does not generate power when the engine is shut down, it is known to provide an electrically driven auxiliary oil pump, which will either continuously maintain the hydraulic supply or at least provide a sufficient oil flow at the required pressure when necessary during inactivity of the internal combustion engine in order to enable a hydraulic shifting of the automatic transmission and, if necessary, make the supply of additional aggregates or components.
From DE 101 62 973 A1 an arrangement is known for controlling the actuation of an oil supply, which has an electrical oil pump in addition to a mechanical oil pump. Both oil pumps feed a hydraulic control unit for the purpose of driving an automatic transmission and a clutch positioned between the internal combustion engine and the transmission. In order to ensure the availability of a sufficient hydraulic pressure and a respective oil flow volume, which are necessary to shift the automatic transmission and the clutch during shutdown of the internal combustion engine, the electrical oil pump is activated based on predetermined criteria when a speed limit value of the internal combustion engine and/or the hydraulic pressure of the mechanical oil pump related thereto are under-exceeded. Once the internal combustion engine is again started and its rotational speed is above a second rotational speed limit, the electrical oil pump is again switched off.
From U.S. Pat. Nos. 5,474,428 A and 6,390,947 B1, as well as EP 1,223,365 A2, similar arrangements are known where an electrical oil pump, in connection with a start/stop function, always ensures that a hydraulic pressure, which is sufficient to shift the transmission, is maintained and/or quickly built up when the internal combustion engine is switched off.
In oil pumps that are mechanically driven by the internal combustion engine, the pump output is at least approximately proportional to its rotational speed, because of the usually rigid coupling to the rotational speed of the internal combustion engine. Instead, the pressure and/or volume flow required for shifting the automatic transmission and a drive clutch is nearly constant and only slightly increases when the transmission receives a normal supply of lubricant by way of the same pump, while the rotational speed or travel speed are increased. Since the mechanically driven oil pump is operated separately for the most part during normal driving operation and an electrical oil pump is activated at best when the rotational speeds of the internal combustion engine are particularly low, the mechanical oil pump must be designed such that it will deliver a sufficient pump output even at low rotational speeds of the internal combustion engine. At high rotational speeds, this would cause an unnecessarily high supply output and would have a corresponding energy loss as a consequence.
The internal combustion engine is generally coupled to an automatic transmission by way of a clutch element that is generally also referred to as a starting element, in particular a friction clutch. So-called wet or oil-cooled starting elements can preferably be used therein. In order to separate the clutches provided in automatic transmissions for the synchronization of transmission components, these starting elements will be referred to as drive clutches. It is unimportant if the drive clutch is structurally separated from the transmission. What is important with regard to its function is that it should transmit the torque of the internal combustion engine to the transmission in a controlled manner, wherein an intermediate position in which the clutch slips is available, in addition to the basic positions of “disengaged” and “fully engaged”.
Although relatively moderate loads occur on the drive clutch when shifting the transmission while driving, the clutch regularly works during starting operations at least in automatic transmissions with fixed gear ratios with slippage until the vehicle speed, the selected gear ratio and the rotational speed of the internal combustion engine enable complete engagement of the clutch. This is the reason why the demand for the coolant supplied to the vehicle clutch is particularly high mostly during starting operations.
It is not feasible to increase the rotational speed of the internal combustion engine to increase the pump output of a mechanical oil pump driven by the internal combustion engine, because the power loss of the drive clutch and also the need for cooling oil are increased directly proportional to the rotational speed of the internal combustion engine.
A design of the oil pump that is sufficient for performing the starting operation has as a result high rotational speeds of the internal combustion engine, an unnecessarily high pump output is available and accordingly great energetic losses are produced, which are reflected in increased fuel consumption as well as increased emission values.
However, even though it is basically conceivable to disengage the rotational drive from the internal combustion engine to the mechanically driven oil pump, for example, by way of an adjustable transmission, or to decrease the pump output of the oil pump when required, for example, with pumps having adjustable output volumes, and thus to reduce the energy losses, these solutions are mechanically quite complex and therefore expensive and prone to failure.
JP 2001 074 130 A describes a hydraulically shiftable transmission with an electrically driven oil pump for supplying a hydraulic control unit. A mechanically driven oil pump feeds an oil coolant flow. A bypass line, which is designed to ensure that the hydraulic control unit is supplied by the mechanically driven oil pump in the event of a failure of the electrical oil pump, is installed between each of the two oil cycles, which are both supplied from the same reservoir. A check valve prevents the oil flow of the electrical oil pump from being fed into the oil cycle provided for cooling.
In this technical solution, the mechanical oil pump must also be configured for the maximum pump output, necessary for starting conditions. In addition, the continuous operation of the electrical oil pump for supplying the hydraulic control unit causes substantial energy losses during the conversion of mechanical into electrical energy and back into mechanical energy.
In this context, it is an object of the invention to disclose a method and an arrangement for controlling the oil supply of an automatic transmission and a starting element, which can ensure a sufficient supply of oil to a hydraulic control unit of the automatic transmission and/or the starting element, in particular an oil-cooled friction clutch, both during operation and inoperation of the internal combustion engine, with the aid of an oil pump that can be mechanically driven by the internal combustion engine and a second oil pump that can be electrically driven. In addition, the cooling oil supply of the starting element is guaranteed according to the method by providing a low-pressure oil flow by way of the electrically driven oil pump so that at least an oil flow for cooling the starting element is available during the starting operation.